Mike Snitzer had a patch in his device-mapper tree:
dm mpath: fix lock order inconsistency in multipath_ioctl
(2014-05-14
16:12:17 -0400)
I was sort of hoping that would address this issue.
Can you check?

Hello Hannes,
Is it possible that that patch already got included in v3.15-rc6 and
hence that that patch was included in my test ?
$ git log v3.15-rc5..v3.15-rc6 | grep 'dm mpath: fix lock order
inconsistency in multipath_ioctl'
dm mpath: fix lock order inconsistency in multipath_ioctl
dm mpath: fix lock order inconsistency in multipath_ioctl